1. Volume 24, Issue 2, Pages 170-175 (January 2014)
Gamete Attachment Requires GEX2 for Successful Fertilization in Arabidopsis 
Toshiyuki Mori, Tomoko Igawa, Gen Tamiya, Shin-ya Miyagishima, Frédéric Berger 
Current Biology 
Volume 24, Issue 2, Pages (January 2014)
DOI: /j.cub
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2. Figure 1 Identification of the New Gamete Fusion-Defective Mutant Y47
(A) A gamete fusion-defective line, Y47, occasionally showed sperm nucleus (sn) signals from unfused sperm cells in ovules (arrow). The area indicated by the arrow is magnified in the inset. In most ovules showing an snRFP signal pair, no morphological changes occurred with the central cell nucleus (ccn) (left ovule), while proliferating endosperm nuclei (enn) were detected in ovules with no snRFP signals (right ovule). The scale bar represents 50 μm.
(B and C) Time-lapse imaging of normal and impaired double fertilization in live Arabidopsis reproductive tissues.
(B) In WT plants, one of the sperm nuclei initiated karyogamy with the central cell nucleus within 1 hr after sperm entering the embryo sac (upper arrowhead in the third and fourth panels of B), and the first karyokinesis of endosperm nuclei took place within 5 hr after completion of karyogamy (the fifth and last panels of B).
(C) In contrast, Y47 sperm nuclei (arrowhead) remained unfused even at more than 6 hr after sperm entrance (right panel).
(D) Ovules showing an snRFP signal were counted. Three to six siliques at 1 day after pollination (DAP) were collected from two to three individuals for each crossing group.
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3. Figure 2
Detection of Mutations Causing Male Sterility in gex2 Mutants and Characterization of GEX2 Structure
(A) Gene structure of Arabidopsis GEX2. Exons and introns are represented with vertical boxes and horizontal lines, respectively. The newly determined GEX2 structure in this study is drawn in black, while that of the gene previously reported by Engel et al. is shown in gray [8]. The position of the point mutation in the Y47 line is indicated. In the gex2-2 line, at least two copies of inverted T-DNA repeat are found in the third exon. The arrows represent primer positions used for T-DNA detection.
(B) PCR assays to detect gex2 mutations. Because a point mutation in gex2-1 lines prevents proper mRNA splicing, longer transcripts were detected in both heterozygous and homozygous gex2-1 flowers (top panel). Only two sets of primers, f4-LB and r2-LB, detected the T-DNA fragments in −/gex2-2 plants (bottom left panel). In RT-PCR assays, no functional GEX2 expression was detected, whereas GCS1 expression was obvious in −/gex2-2 flowers (bottom right panel).
(C) Phenotypes of gex2-2 plants. gex2-2 transmission was evaluated with that of the Kan-resistant gene (KanR) contained in the T-DNA insert (top panel). Offspring seeds were collected from two to three individuals for each crossing group. −/gex2-2+/snGFP plants, in which sperm nuclei are GFP labeled, were obtained (bottom left). Frequent snGFP signals were detected in WT ovules after pollination with −/gex2-2+/snGFP pollen (bottom right). The frequency (percentage) of snGFP-positive ovules is shown with the ±SD in parentheses (n = 333). Scale bars represent 15 μm (bottom left) and 50 μm (bottom right).
(D) Characterization of the GEX2 protein. GEX2 protein is composed of an N-terminal signal sequence (SS), filamin repeat domains (FLMN), and a C-terminal transmembrane domain (top). Amino acid positions of those domains above are indicated. The hydropathy profile of GEX2 detected remarkable peaks (shaded black) in the N- and C-terminal regions, reflecting the SS and TM domains, respectively (bottom).
(E–G) pGEX2::gGEX2-GFP expression in −/gex2-1 pollen. (E) and (F) are an identical field group and are merged in (G). Sperm nuclei (sn) are specifically labeled with HTR10-RFP (E). The GEX2-GFP signals were detected specifically in sperm cells (F). Tailed GFP signals (arrow) were observed besides sn-surrounding ones (arrowhead). Scale bar represents 10 μm.
(H and I) GEX2-GFP signals were also detected in sperm cells migrating in a pollen tube of the GEX2-GFP-expressing line (H, arrowheads). Similar sn-surrounding signals, derived from endomembrane, were also detected in a GCS1-GFP-expressing line (I, arrowheads). Scale bar represents 10 μm.
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4. Figure 3 Abnormal Seed Development in gex2 Plants
(A) Almost all seeds showed normal development in selfed WT plants, while undeveloped ovules (arrows) and aborted seeds (arrowheads) were frequently detected in −/gex2-1 and −/gex2-2 plants.
(B) Normally developing seeds (green), aborted seeds (pink), and undeveloped ovules (brown) were counted. Three to five siliques were collected from each of three individuals at ∼10 DAP for each crossing group.
(C) Various fertilization patterns in the gex2-1 line. Ovules at 2 DAP were observed and categorized as follows: normal development showing proliferation of both endosperm (enn) and embryo (emn) nuclei (left), development of the embryo alone (center) or the endosperm alone (right), and no development (not shown). The remnants of the snRFP signal were occasionally detected in the incompletely developing ovules described above (sn). The percentage of each development type observed in three individuals (except for “no development”) is shown with the ±SD in parentheses (n = 345). Scale bar represents 50 μm.
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5. Figure 4 Molecular Function Analyses of GEX2
(A) To assess gamete attachment, we prepared an egg cell marker line expressing both enRFP and emGFP. After enzymatic treatment, the egg cell (pe) was plasmolyzed and assumed a round shape.
(B–D) When snRFP-positive ovules after pollination with −/gex2 or +/gcs1 pollen were observed, unfused sperm cells (sn) attached to (B and C) or detached from (D) the egg cell were detected.
(E) ccGFP-expressing ovules were also enzymatically treated to induce plasmolyzed central cells (pc). Scale bar represents 50 μm.
(F–H) Similar sperm behaviors were also detected in ccGFP-expressing ovules. Scale bar represents 25 μm. Insets show single sperm cells attached to (C and G) or detached from (D and H) the female gamete. The percentages of each class of phenotype in gex2-1 are shown with the ±SD in parentheses in (B)–(D) (n = 39) and (F)–(H) (n = 40). Each experiment was repeated two or three times.
(I) Percentage of ovules with detached sperm cells (shown in D and H) in gex2-1 and gcs1. n∗ represents the total number of ovules showing completely plasmolyzed egg or central cell and arrested sperm cells.
(J) Ka/Ks ratios on GEX2 and GCS1 were calculated between A. thaliana and A. lyrata using a sliding window analysis. The window size corresponds to 10% of total length analyzed, and the window slid with a step size of 50% of the window. The averages of Ka/Ks ratio on GEX2 and GCS1 are 0.17 and 0.04, respectively. The positions of FLMNs, deduced by three prediction software tools utilizing their large databases (PROSITE, HMMPfam, and Gene3D; are shown.
Current Biology  , DOI: ( /j.cub )
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